The two main sources of atmospheric ultrafine particles are combustion (particularly from vehicles) and atmospheric new particle formation. Using models to simulate both sources of size resolved aerosol particles, we will present predictions for the balance of these sources in near-road, urban, and rural continental environments. A key input of these models is the number and surface area distribution from on road vehicles. Progress in a critical review of existing literature on size-resolved activity-based emission factors will be presented. Simulation results will be contrasted with measurements from the rural Midwest U.S., from Mexico City, and from other U.S. locations. During periods where combustion emissions dominate, a carbon dioxide-based emission factors for ultrafine particles can be developed or evaluated from ambient particle measurements. For example, in March 2006, during the MILAGRO field campaign in Mexico City, observations at the T0 location could often be characterized by morning conditions with high particle mass concentrations, low mixing heights, and good correlation between particle number and carbon dioxide, indicative that particle number is controlled by primary emissions. Using this correlation between particle number and carbon dioxide, an average number based-emission factor of 3.4 x 10¹³#/vehicle-km has been determined. In the afternoon, the CO₂ level drops during ventilation of the daily polluted layer, and the coupling between CO₂ and particle number breaks down, with particle number sometimes increasing as CO₂ decreases. New particle formation events were observed both simultaneous to and separately from the afternoon ventilation.